The ongoing feud between physicists and philosophers cuts to the heart of what science can tell us about the nature of reality.

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Editor’s Note: Shortly before his death last August at the age of 79, the noted physicist and public intellectual Victor Stenger worked with two co-authors to pen an article for Scientific American. In it Stenger and co-authors address the latest eruption of a long-standing historic feud, an argument between physicists and philosophers about the nature of their disciplines and the limits of science. Can instruments and experiments (or pure reason and theoretical models) ever reveal the ultimate nature of reality? Does the modern triumph of physics make philosophy obsolete? What philosophy, if any, could modern theoretical physicists be said to possess? Stenger and his co-authors introduce and address all these profound questions in this thoughtful essay and seek to mend the growing schism between these two great schools of thought. When physicists make claims about the universe, Stenger writes, they are also engaging in a grand philosophical tradition that dates back thousands of years. Inescapably, physicists are philosophers, too. This article, Stenger’s last, appears in full below.

In April 2012 theoretical physicist, cosmologist and best-selling author Lawrence Krauss was pressed hard in an interview with Ross Andersen for The Atlantic titled “Has Physics Made Philosophy and Religion Obsolete?” Krauss's response to this question dismayed philosophers because he remarked, “philosophy used to be a field that had content,” to which he later added,

“Philosophy is a field that, unfortunately, reminds me of that old Woody Allen joke, “those that can't do, teach, and those that can't teach, teach gym.” And the worst part of philosophy is the philosophy of science; the only people, as far as I can tell, that read work by philosophers of science are other philosophers of science. It has no impact on physics whatsoever, and I doubt that other philosophers read it because it's fairly technical. And so it's really hard to understand what justifies it. And so I'd say that this tension occurs because people in philosophy feel threatened—and they have every right to feel threatened, because science progresses and philosophy doesn't.”

Later that year Krauss had a friendly discussion with philosopher Julian Baggini in The Observer, an online magazine from The Guardian. Although showing great respect for science and agreeing with Krauss and most other physicists and cosmologists that there isn’t “more stuff in the universe than the stuff of physical science,” Baggini complained that Krauss seems to share “some of science’s imperialist ambitions.” Baggini voices the common opinion that “there are some issues of human existence that just aren’t scientific. I cannot see how mere facts could ever settle the issue of what is morally right or wrong, for example.”

Krauss does not see it quite that way. Rather he distinguishes between “questions that are answerable and those that are not,” and the answerable ones mostly fall into the “domain of empirical knowledge, aka science.” As for moral questions, Krauss claims that they only be answered by “reason...based on empirical evidence.” Baggini cannot see how any “factual discovery could ever settle a question of right and wrong.”

Nevertheless, Krauss expresses sympathy with Baggini’s position, saying, “I do think philosophical discussion can inform decision-making in many important ways—by allowing reflections on facts, but that ultimately the only source of facts is via empirical exploration.”

Noted philosophers were upset with The Atlantic interview, including Daniel Dennett of Tufts University who wrote to Krauss. As a result, Krauss penned a more careful explication of his position that was published in Scientific American in 2014 under the title “The Consolation of Philosophy.” There he was more generous to philosophy's contribution to the enrichment of his own thinking, although he conceded little of his basic position:

“As a practicing physicist...I, and most of the colleagues with whom I have discussed this matter, have found that philosophical speculations about physics and the nature of science are not particularly useful, and have had little or no impact upon progress in my field. Even in several areas associated with what one can rightfully call the philosophy of science I have found the reflections of physicists to be more useful.”

Krauss is not alone among physicists in his disdain for philosophy. In September 2010 physicists Stephen Hawking and Leonard Mlodinow published a shot heard round the world—and not just the academic world. On the first page of their book, The Grand Design, they wrote: “Philosophy is dead” because “philosophers have not kept up with modern developments in science, particularly physics. Scientists have become the bearers of the torch of discovery in our quest for knowledge.”

The questions that philosophy is no longer capable of handling (if it ever was) include: How does the universe behave? What is the nature of reality? Where did all this come from? Did the universe need a creator? According to Hawking and Mlodinow, only scientists—not philosophers—can provide the answers.

Famous astrophysicist and science popularizer Neil deGrasse Tyson has joined the debate. In an interview on the Nerdist podcast in May 2014 Tyson remarked, “My concern here is that the philosophers believe they are actually asking deep questions about nature. And to the scientist it's, ‘What are you doing? Why are you concerning yourself with the meaning of meaning?’” His overall message was clear: science moves on; philosophy stays mired, useless and effectively dead.

Needless to say, Tyson also has been heavily criticized for his views. His position can be greatly clarified by viewing the video of his appearance in a forum at Howard University in 2010, where he was on the stage with biologist Richard Dawkins. Tyson's argument is straightforward and is the same as expressed by Krauss: Philosophers from the time of Plato and Aristotle have claimed that knowledge about the world can be obtained by pure thought alone. As Tyson explained, such knowledge cannot be obtained by someone sitting back in an armchair. It can only be gained by observation and experiment. Richard Feynman had once expressed a similar opinion about “armchair philosophers.” Dawkins agreed with Tyson, pointing out that natural selection was discovered by two naturalists, Charles Darwin and Alfred Russel Wallace, who worked in the field gathering data.

What we are seeing here is not a recent phenomenon. In his 1992 book Dreams of a Final Theory, Nobel laureate Steven Weinberg has a whole chapter entitled “Against Philosophy.” Referring to the famous observation of Nobel laureate physicist Eugene Wigner about “the unreasonable effectiveness of mathematics,” Weinberg puzzles about “the unreasonable ineffectiveness of philosophy.”

Weinberg does not dismiss all of philosophy, just the philosophy of science, noting that its arcane discussions interest few scientists. He points out the problems with the philosophy of positivism, although he agrees that it played a role in the early development of both relativity and quantum mechanics. He argues that positivism did more harm than good, however, writing, “The positivist concentration on observables like particle positions and momenta has stood in the way of a ‘realist’ interpretation of quantum mechanics, in which the wave function is the representative of physical reality.”

Perhaps the most influential positivist was late 19th-century philosopher and physicist Ernst Mach, who refused to accept the atomic model of matter because he could not see atoms. Today we can see atoms with a scanning tunneling microscope but our models still contain unseen objects such as quarks. Philosophers as well as physicists no longer take positivism seriously, and so it has no remaining influence on physics, good or bad.

Nevertheless, most physicists would agree with Krauss and Tyson that observation is the only reliable source of knowledge about the natural world. Some, but not all, incline toward instrumentalism, in which theories are merely conceptual tools for classifying, systematizing and predicting observational statements. Those conceptual tools may include nonobservable objects such as quarks.

Until very recently in history no distinction was made between physics and natural philosophy. Thales of Miletus (circa 624–546 B.C.) is generally regarded as the first physicist as well as the first philosopher of the Western tradition. He sought natural explanations for phenomena that made no reference to mythology. For example, he explained earthquakes to be the result of Earth resting on water and being rocked by waves. He reasoned this from observation, not pure thought: Land is surrounded by water and boats on water are seen to rock. Although Thales’ explanation for earthquakes was not correct, it was still an improvement over the mythology that they are caused by the god Poseidon striking the ground with his trident.

Thales is famous for predicting an eclipse of the sun that modern astronomers calculate occurred over Asia Minor on May 28, 585 B.C. Most historians today, however, doubt the truth of this tale. Thales’ most significant contribution was to propose that all material substances are composed of a single elementary constituent—namely, water. Whereas he was (not unreasonably) wrong about water being elementary, Thales’ proposal represents the first recorded attempt, at least in the West, to explain the nature of matter without the invocation of invisible spirits.

Thales and other Ionian philosophers who followed espoused a view of reality now called material monism in which everything is matter and nothing else. Today this remains the prevailing view of physicists, who find no need to introduce supernatural elements into their models, which successfully describe all their observations to date.

The rift to which Tyson was referring formed when physics and natural philosophy began to diverge into separate disciplines in the 17th century after Galileo and Newton introduced the principles that describe the motion of bodies. Newton was able to derive from first principles the laws of planetary motion that had been discovered earlier by Kepler. The successful prediction of the return of Halley’s Comet in 1759 demonstrated the great power of the new science for all to see.

The success of Newtonian physics opened up the prospect for a philosophical stance that became known as the clockwork universe, or alternatively, the Newtonian world machine. According to this scheme, the laws of mechanics determine everything that happens in the material world. In particular, there is no place for a god who plays an active role in the universe. As shown by the French mathematician, astronomer and physicist Pierre-Simon Laplace, Newton's laws were in themselves sufficient to explain the movement of the planets throughout previous history. This led him to propose a radical notion that Newton had rejected: Nothing besides physics is needed to understand the physical universe.

Whereas the clockwork universe has been invalidated by the Heisenberg uncertainty principle of quantum mechanics, quantum mechanics remains devilishly hard to interpret philosophically. Rather than say physics “understands” the universe, it is more accurate to say that the models of physics remain sufficient to describe the material world as we observe it to be with our eyes and instruments.

In the early part of the 20th century almost all the famous physicists of the era—Albert Einstein, Niels Bohr, Erwin Schrödinger, Werner Heisenberg, Max Born, among others—considered the philosophical ramifications of their revolutionary discoveries in relativity and quantum mechanics. After World War II, however, the new generation of prominent figures in physics—Richard Feynman, Murray Gell-Mann, Steven Weinberg, Sheldon Glashow and others—found such musings unproductive, and most physicists (there were exceptions in both eras) followed their lead. But the new generation still went ahead and adopted philosophical doctrines, or at least spoke in philosophical terms, without admitting it to themselves.

For example, when Weinberg promotes a “realist” interpretation of quantum mechanics, in which “the wave function is the representative of physical reality,” he is implying that the artifacts theorists include in their models, such as quantum fields, are the ultimate ingredients of reality. In a 2012 Scientific American article theoretical physicist David Tong goes even further than Weinberg in arguing that the particles we actually observe in experiments are illusions and those physicists who say they are fundamental are disingenuous:

“Physicists routinely teach that the building blocks of nature are discrete particles such as the electron or quark. That is a lie. The building blocks of our theories are not particles but fields: continuous, fluidlike objects spread throughout space.”

This view is explicitly philosophical, and accepting it uncritically makes for bad philosophical thinking. Weinberg and Tong, in fact, are expressing a platonic view of reality commonly held by many theoretical physicists and mathematicians. They are taking their equations and model as existing on one-to-one correspondence with the ultimate nature of reality.

“Platonism is the view that there exist [in ultimate reality] such things as abstract objects—where an abstract object is an object that does not exist in space or time and which is therefore entirely nonphysical and nonmental. Platonism in this sense is a contemporary view. It is obviously related to the views of Plato in important ways but it is not entirely clear that Plato endorsed this view as it is defined here. In order to remain neutral on this question, the term ‘platonism’ is spelled with a lower-case ‘p.’”

We will use platonism with a lower-case “p” here to refer to the belief that the objects within the models of theoretical physics constitute elements of reality, but these models are not based on pure thought, which is Platonism with a capital “P,” but fashioned to describe and predict observations.

Many physicists have uncritically adopted platonic realism as their personal interpretation of the meaning of physics. This not inconsequential because it associates a reality that lies beyond the senses with the cognitive tools humans use to describe observations.

In order to test their models all physicists assume that the elements of these models correspond in some way to reality. But those models are compared with the data that flow from particle detectors on the floors of accelerator labs or at the foci of telescopes (photons are particles, too). It is data—not theory—that decides if a particular model corresponds in some way to reality. If the model fails to fit the data, then it certainly has no connection with reality. If it fits the data, then it likely has some connection. But what is that connection? Models are squiggles on the whiteboards in the theory section of the physics building. Those squiggles are easily erased; the data can’t be.

In his Scientific American article Krauss reveals traces of platonic thinking in his personal philosophy of physics, writing:

“There is a class of philosophers, some theologically inspired, who object to the very fact that scientists might presume to address any version of this fundamental ontological issue. Recently one review of my book [A Universe from Nothing] by such a philosopher.... This author claimed with apparent authority (surprising because the author apparently has some background in physics) something that is simply wrong: that the laws of physics can never dynamically determine which particles and fields exist and whether space itself exists or more generally what the nature of existence might be. But that is precisely what is possible in the context of modern quantum field theory in curved spacetime.”

The direct, platonic, correspondence of physical theories to the nature of reality, as Weinberg, Tong and possibly Krauss have done, is fraught with problems: First, theories are notoriously temporary. We can never know if quantum field theory will not someday be replaced with another more powerful model that makes no mention of fields (or particles, for that matter). Second, as with all physical theories, quantum field theory is a model—a human contrivance. We test our models to find out if they work; but we can never be sure, even for highly predictive models like quantum electrodynamics, to what degree they correspond to “reality.” To claim they do is metaphysics. If there were an empirical way to determine ultimate reality, it would be physics, not metaphysics; but it seems there isn't.

In the instrumentalist view we have no way of knowing what constitutes the elements of ultimate reality. In that view reality just constrains what we observe; it need not exist in one-to-one correspondence with the mathematical models theorists invent to describe those observations. Furthermore, it doesn’t matter. All these models have to do is describe observations, and they don’t need metaphysics to do that. The explanatory salience of our models may be the core of the romance of science but it plays second chair to its descriptive and predictive capacity. Quantum mechanics is a prime example of this because of its unambiguous usefulness despite lacking an agreed-on philosophical interpretation.

Thus, those who hold to a platonic view of reality are being disingenuous when they disparage philosophy. They are adopting the doctrine of one of the most influential philosophers of all time. That makes them philosophers, too.

Now, not all physicists who criticize philosophers are full-fledged platonists, although many skirt close to it when they talk about the mathematical elements of their models and the laws they invent as if they are built into the structure of the universe. Indeed, the objections of Weinberg, Hawking, Mlodinow, Krauss, and Tyson are better addressed to metaphysics and fail to show sufficient appreciation, in our view, for the vital contributions to human thought that persist in fields like ethics, aesthetics, politics and, perhaps most important, epistemology. Krauss pays these important topics some lip service, but not very enthusiastically.

Of course, Hawking and Mlodinow write mostly with cosmological concerns in mind—and where metaphysical attempts to grapple with the question of ultimate origins trespass on them, they are absolutely correct. Metaphysics and its proto-cosmological speculations, construed as philosophy, were in medieval times considered the handmaiden of theology. Hawking and Mlodinow are saying that metaphysicians who want to deal with cosmological issues are not scientifically savvy enough to contribute usefully. For cosmological purposes, armchair metaphysics is dead, supplanted by the more informed philosophy of physics, and few but theologians would disagree.

Krauss leveled his most scathing criticisms at the philosophy of science, and we suggest that it would have been more constructive had he targeted certain aspects of metaphysics. Andersen, for The Atlantic, interviewed him on whether physics has made philosophy and religion obsolete. And although it hasn't done so for philosophy, it has for cosmological metaphysics (and the religious claims that depend on it, such as the defunct Kalm cosmological argument begging the necessity of a creator). Surely Krauss had metaphysical attempts to speculate about the universe at least partially in mind, given that the interview addressed his book on cosmology.

Whatever may be the branches of philosophy that deserve the esteem of academics and the public, metaphysics is not among them. The problem is straightforward. Metaphysics professes to be able to hook itself to reality—to legitimately describe reality—but there's no way to know if it does.

So, although the prominent physicists we have mentioned, and the others who inhabit the same camp, are right to disparage cosmological metaphysics, we feel they are dead wrong if they think they have completely divorced themselves from philosophy. First, as already emphasized, those who promote the reality of the mathematical objects of their models are dabbling in platonic metaphysics whether they know it or not. Second, those who have not adopted platonism outright still apply epistemological thinking in their pronouncements when they assert that observation is our only source of knowledge.

Hawking and Mlodinow clearly reject platonism when they say, “There is no picture- or theory-independent concept of reality.” Instead, they endorse a philosophical doctrine they call model-dependent realism, which is “the idea that a physical theory or world picture is a model (generally of a mathematical nature) and a set of rules that connect the elements of the model to observations.” But they make it clear that “it is pointless to ask whether a model is real, only whether it agrees with observations.”

We are not sure how model-dependent realism differs from instrumentalism. In both cases physicists concern themselves only with observations and, although they do not deny that they are the consequence of some ultimate reality, they do not insist that the models describing those observations correspond exactly to that reality. In any case, Hawking and Mlodinow are acting as philosophers—epistemologists at the minimum—by discussing what we can know about ultimate reality, even if their answer is “nothing.”

All of the prominent critics of philosophy whose views we have discussed think very deeply about the source of human knowledge. That is, they are all epistemologists. The best they can say is they know more about science than (most) professional philosophers and rely on observation and experiment rather than pure thought—not that they aren’t philosophizing. Certainly, then, philosophy is not dead. That designation is more aptly applied to pure-thought variants like those that comprise cosmological metaphysics.

Victor J. Stenger (1935–2014) was emeritus professor of physics at the University of Hawaii and adjunct professor of philosophy at the University of Colorado. He is author of The New York Times bestseller, God:The Failed Hypothesis: How Science Shows That God Does Not Exist. His latest book is God and the Multiverse: Humanity’s Expanding View of the Cosmos.

James A. Lindsay has a PhD in mathematics and is author of God Doesn't; We Do: Only Humans Can Solve Human Challenges and Dot, Dot, Dot: Infinity Plus God Equals Folly.

Peter Boghossian is an assistant professor of philosophy at Portland State University and an affiliate faculty member at Oregon Health & Science University in the Division of General Internal Medicine. He is author of the bestseller, A Manual for Creating Atheists.

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